1. The Field of the Invention
The present invention relates to angular rate sensors or gyroscopes, and more particularly to angular rate sensors configured to measure the rate of an angular rotation in a method for detecting a magnitude of a Coriolis force generated by interactions between a vibrating motion of a mass on a piezoelectric substrate and a rotary motion of the piezoelectric substrate by converting the Coriolis force into a voltage due to a piezoelectric effect.
2. Description of the Prior Art
There has been increasing demand for requiring angular rate information, which is also sometimes called angular velocity information not only in the field of automotive products where, for example, inertial navigation and guidance systems, air-bag systems, and anti-skid systems use angular rate information for running, but also in other fields, for example, in the field of still or video cameras where a manual blur correcting system can be found also in the field of medical products, surgical tools, and body movement monitoring apparatus, all of these require angular rate information. Generally a measuring device for obtaining angular rate information is called a gyroscope, gyro sensor, angular rate sensor, angular velocity sensor and so on. Recently, an inexpensive oscillatory type angular rate sensor has been developed in order to be used in the above mentioned fields.
There has been proposed in recent years an angular rate sensor utilizing a piezoelectric element or substrate made of quartz or lithium tantalite capable of providing a smaller and less expensive angular rate sensor.
One known prior art embodiment of an angular rate sensor utilizing a single crystalline piezoelectric element typically has a pair of arms which are joined and fixed at their individual end portions by a root member to form a tuning folk oscillator, as shown FIG. 45. Such an angular rate sensor is disclosed by, for example, U.S. Pat. No. 5,719,460. Although a angular rate sensor disclosed in U.S. Pat. No. 5,719,460 has more complex structures, a basic operation principle is as follows. A set of drive electrodes are affixed to one of the arms of the tuning folk oscillator for driving the tuning folk oscillator in a direction of a principal plane at a resonant frequency due to piezoelectric effect which convert electric energy into mechanical deformation energy and vice versa. Thus, the driving electrodes are electrically driven by an external oscillator circuit. A monitoring electrode, and a sensing electrode are affixed to the other arm. The monitoring electrode serves to detect the oscillation amplitude generated by the oscillator circuit. The sensing electrode serves to detect the stress caused by the Coriolis force acting on the tuning folk and being generated by a rotating motion of the angular rate sensor. In more detail, in order to keep the oscillation amplitude of the tuning folk constant, the electric charge generated on the monitor electrode due to the piezoelectric effect in the direction of the principal plane is amplified by the external circuit and then compared with a reference signal to control the oscillator circuit. On the other hand, the sensing electrode detects a signal generated by the Coriolis force, which is amplified synchronously with the signal detected by the monitoring electrode.
Angular rate sensors of this type suffer from an inherent performance limitation. For example, quartz is typically single crystalline piezoelectric material composed of arrayed single crystals of silicon oxide (SiO2). Since silicon (Si) has a positive polarity and oxide (O2) has negative polarity, symmetric arraying silicon (Si) and oxide (O2) leads to establish electric neutrality. However, if a stress is applied to the silicon oxide (SiO2) single crystalline piezoelectric material, the electric symmetry is broken and electric charge is generated.
In FIG. 46, the individual axes of a quartz crystal are shown. As shown, X-axes or electric axes are defined by the ridge lines and a Z-axis or optical axis is defined by an axis being perpendicular to the plane extended by the X-X axes. A single crystalline piezoelectric material such as quartz exhibits specific piezoelectric characteristic and has specific polarities with respect to the crystal axes which depend on the array of molecules of the crystalline piezoelectric material.
The oscillatory type angular rate sensor detects a rotating motion of the angular rate sensor by detecting the Coriolis force acting at a right angle with respect to the direction of the oscillations of the tuning folks. Thus, the angular rate sensor is required to have piezoelectric characteristics for orthogonal two axes and a means for applying the oscillations and a means for detecting the deformations at a right angle to the applied oscillations due to the Coriolis force. Although a single crystalline tuning folk oscillator is cut from a piezoelectric material is optimized in a view of the polarities on which the sensitivity of the sensor depends, it is not easy to obtain the high sensitivity of both directions corresponding to the oscillatory direction of the tuning folk and the direction of the deformation due to the Coriolis force. Furthermore, the pair of arms and the root member being components of the tuning folks are susceptible to external shock and external vibrations that occur at frequencies close to the arm vibrating frequency. Such disturbances may influence the vibrating structure and produce erroneous results.
Another known prior art embodiment of an angular rate sensor utilizes a surface acoustic wave (SAW) on a piezoelectric substrate, and more particularly to a micro-electro-mechanical (MEM) angular rate sensor that includes a surface acoustic wave resonator (SAWR) and a surface acoustic wave sensors (SAWS).
The basic operating principle of the angular rate sensor utilizing the surface acoustic waves (SAWs) is as follows. When two progressive SAWs which propagate on the same axis but in the opposite directions each other are added, a standing wave is generated on an elastic material surface, where the elastic material is composed of the particles such as molecules. If the standing wave is Rayleigh wave, the wave motion of the surface particles is distributed such that each particle undergoes a periodic motion whose orbit is ellipse in the plane which is orthogonal to the surface of the piezoelectric material and parallel to the propagating direction of the SAWs. Some particles seem to be stationary since an elliptical orbit is collapsed to a nodal point in the plane which is orthogonal to the surface of the piezoelectric material and parallel to the propagating direction of the SAWs. At the nodal point, the particles vibrate in the tangential direction. The Coriolis force acts on the vibrating particles. In order for the angular rate to be detected, the action of the Coriolis force on the particles has to be near the surface of the material. This occurs due to the distribution of the wave motion on the elastic material surface. The secondary wave caused by the Coriolis force can then be detected and the angular rate is quantified.
If the particle of a mass m, a member composing the elastic material, undergoes a vibrating motion , an angular rotation  perpendicular to the direction of the vibrating motion  causes a Coriolis force  perpendicular to the directions of both the vibrating motion  and the angular rotation . Where the vibrating motion , the angular rotation , and the Coriolis force  each has three components, in general. Therefore, the effect of the Coriolis force
    =      2    ⁢    m    ⁢                  ⁢          ν      ρ        ×          ω      ρ      is a measure of the rate of the angular rotation . Where a symbol “x” in the above equation represents an external product.
When the angular rate sensor utilizing the surface acoustic waves on a surface of the elastic substrate is rotated, i.e., the elastic substrate is rotated, the Coriolis force is applied to particles vibrating in the standing wave. The direction of the Coriolis force alternates even if the angular rate is constant in time because the velocity of the particles change temporally depending on the phase of the wave to which the particle contributes. The alternating force generates a secondary SAW in the direction orthogonal to the primary standing wave. As the Coriolis force is proportional to both particle vibration velocity and the angular rate of the substrate, the amplitude of the secondary wave is also proportional to the angular rate. The magnitude of the Coriolis force can be measured by detecting the amplitude of the secondary SAW. Therefore, the angular rate can be obtained from the magnitude of the Coriolis force. However, if an attempt of detecting the magnitude of the Coriolis force by averaging over a spatial distribution of the vibration of each particle composing the elastic material is made, it would not be effective because the Coriolis force acting on each particle cancel one another. Therefore, secondary SAWs will never be obtained due to the cancellation.
However, if perturbation masses are arranged on grids at intervals of a wavelength such that all masses are arranged at nodes of the primary standing wave, the Coriolis force acting in the area with the perturbation masses are stronger than the Coriolis force acting on the area without the perturbation masses because the total weight of the particle and the perturbation masses is heavier than that of particle alone, without the perturbation masses. The coherent alternating forces generated at each perturbation mass build up another SAW which propagates in the orthogonal direction to the primary standing wave.
If an elastic material is made of piezoelectric material which converts the elastic deformation into an electric field and the secondary wave generated by the Coriolis force is detected by two detection electrodes, the output voltage is proportional to the amplitude of the secondary wave. It is preferred that each perturbation mass is deposited with a metal forming an electrode which has a higher mass density.
An elastic surface wave gyroscope and a micro-electro-mechanical gyroscope both utilizing the above mentioned operating principle are disclosed in Japanese Unexamined Patent Publication No. 8-334330 to Kurosawa and Higuchi and U.S. Pat. No. 6,516,665 to Varadan et al., as shown in FIGS. 47 and 48. Both the elastic surface wave gyroscope of Kurosawa and Higuchi and the micro-electro-mechanical gyroscope of Varadan et al. Include two pairs of transducers disposed on a piezoelectric substrate with a plurality of metallic dots arranged in an array, and a pair of reflectors. The plurality of metallic dots serves as a proof mass. One pair of transducers generates the primary surface acoustic wave and is called a surface acoustic wave resonator or a driving inter-digital transducer (hereinafter, “driving IDT”). A pair of reflectors is provided on the outsides of the driving IDT and are arranged to effectively generate the primary surface acoustic wave by reflecting the progressive surface wave generated by the driving IDT on one surface of a piezoelectric substrate. The secondary surface acoustic wave generated by the Coriolis force is sensed by another pair of transducers called a surface acoustic wave sensor or detecting inter-digital transducers (hereafter, “detecting IDTs”). The driving IDT and the detecting IDTs are arranged perpendicularly each other. It is preferred that the plurality of metallic dots form square electrodes which are sandwiched by both the driving IDT and the detecting IDTs. Preferably, another pair of reflectors is provided on the outsides of the detecting IDTs and are arranged to effectively generate the secondary surface acoustic wave.
By using SAWs, it becomes possible for the angular rate sensor to have a two-dimensional construction, as it can be manufactured only by forming electrodes deposited on the surface of the piezoelectric materials in a method of production technology for very-large-scale integrated circuits (VLSIs).
FIG. 47 is a diagram of a prior art embodiment of an angular rate sensor disclosed in Japanese Unexamined Patent Publication No. 8-334330 to Kurosawa and Higuchi showing the driving. IDT J2, the pairs of reflectors J3, J4, J7, and J8, the pair of the detecting IDTs J5, J6, and metallic dots J1, each metallic dot serving as a perturbation mass, formed on the surface of the piezoelectric substrate of the angular rate sensor. The driving IDT J2 and the detecting IDTs J5 are formed such that the teeth of the respective comb-shaped electrodes are located in predetermined positions corresponding to the nodes of the elastic surface wave.
The plurality of metallic dots J1 form almost square electrode on the piezoelectric substrate. The electrodes sides are parallel to an x-axis and a y-axis which are mutually orthogonal, as shown in FIG. 47. The driving IDT J2 which is composed of the comb-shaped electrodes is positioned on the side of the square electrodes along the x-axis. The plurality of metallic dots J1 and the driving IST J2 are sandwiched by one of the pairs of reflectors J3, J4. On the surface of the piezoelectric substrate, a first standing wave of elastic surface waves are generated within a region J10 of the surface by causing the driving IDT J2 to generate elastic surface waves propagating in outward directions along the x-axis therefrom and by reflecting these elastic surface waves by the reflectors J3, J4. The plurality of metallic dots J1 is disposed on the surface within the region J10. The reflectors J3, J4 are separated from each other by an integral distance equal to one half of wavelength of the first standing wave. The driving IDT has electrodes spaced apart at a distance equal to one half of the first standing wave.
The detecting IDTs J5, J6 are disposed on the surface of the piezoelectric substrate, separated from one another by the region J8 and disposed orthogonally to the pair of the reflectors J3, J4, i.e., the x-axis. In other words, the detecting IDTs J5, J6 are positioned along the y-axis. The region J10 and the detecting IDTs J5, J6 are sandwiched by another pair of reflectors J7, J8 along the y-axis. The detecting IDTs J5, J6 are configured to sense a second surface acoustic wave and provide an output indicative of the characteristics of the second surface acoustic wave. In order to intensify the effect of the Coriolis force on metallic dots J1, each metallic dot is preferably located at an anti-node of the first standing wave.
FIG. 48 shows a simplified operating principle of the above-mentioned angular rate sensor including the driving IDT J2, the pairs of reflectors J3, J4, J7, and J8, the pair of the detecting IDTs J5, J6, and metallic dots J1. In FIG. 48, a relationship between an amplitude of the first standing wave caused by the driving IDT J2 is shown. Metallic dots J1 have individual metallic dots J11, J12, J13, J14, and J15 as shown in FIG. 48.
If the first surface acoustic wave (SAW) causes metallic dots J1 to oscillate along the x-axis, and the piezoelectric substrate of the angular rate sensor is rotated about the x-axis, the Coriolis force, which is related to the rate of rotation, is detected along the y-axis. The first surface acoustic wave (SAW) on the piezoelectric substrate is generated by applying an alternating current (AC) voltage to the driving IDT J2. The resonant frequency of the first SAW is determined by the distance between the comb-shaped electrodes which constitute the driving IDT J2, and ranged, for example, from 10 MHz to several hundreds MHz. The SAW generated by the driving IDT J2 propagates back and forth between the reflectors J3, J4 and forms a first standing wave in region J10 between the detecting IDTs J5, J6 due to the collective reflection from the reflectors J3, J4. That is, the reflectors J3 and J4 contribute to improve the efficiency of the excitation of the standing wave of the SAW along the x-axis by confining the SAW generated by the driving IDT J2 in the region J10.
Since each of metallic dots J11, J12, J13, J14, and J15 is designed to be arranged on an anti-node of the first standing wave within region J10, particles on which metallic dots J1 are disposed will experience a larger amplitude of vibration in a z-direction, which serves as the reference vibrating motion for the angular rate sensor. The z-axis is defined as a direction orthogonal to both the x-axis and the y-axis.
The metallic dots J1 can be made of a metal film of any metal, such as, for example, gold or aluminum. The metallic dots J1 are subjected to oscillatory motion due to the standing wave excited within the region J10 between the reflectors J3, J4. If metallic dots J1 are too large and too heavy, they will affect the formation of the standing wave. Hence, although the shape of the individual metallic dot is not important, the size, position, thickness, and weight are important, and their relative positions are especially important. When the angular rate sensor is rotated, the Coriolis force acting on the metallic dots J1 produces a second SAW. Thus, the metallic dots J1 are also spaced such that the phases of the second SAW are coherent and superposed to provide a sufficient signal to the detecting IDTs J5, J6.
The metallic dots J1 lie in an x-y plane, in which the x-axis runs from the reflector J3 to another reflector J4 and defines a x direction, and the y-axis runs from the reflector J8 to another reflector J7 and defines a y direction. In this case, λ1 is the wavelength of the first SAW in the x-direction, and λ2 is the wavelength of the second SAW in the y-direction. Since the wave length λ1 in the x-direction and λ2 in the y-direction are different to each other due to the wave velocity being different in the x and y directions, the spacing between the metallic dots J1 in the x and y directions is also different. The metallic dots J1 are spaced with a separation of λ1 in the x-direction and λ2 in the y-direction.
Furthermore, the metallic dots J1 are interlaced in both the x and y directions such that the second surface acoustic waves generated by the Coriolis force are coherently superposed. For example, as shown in FIG. 48, when the Coriolis force acts on the metallic dots J1, the metallic dots J11, J12, J13, J14 vibrate coherently with the inverse phase of the metallic dot J15 since the metallic dot J15 is located from every metallic dots J11, J12, J13, J14 by .lamda..sub.1/2 in the x direction and by .lamda..sub.2/2 in the y direction. So, when the angular rate sensor rotates about the x-axis with rotation rate then each of the metallic dots J1 experiences an acceleration of in the y-direction, where is the velocity vector of the particle on which the individual metallic dot is disposed. The acceleration shows a vector quantity having three components, the first, second, and third component corresponding to the scalar quantity in the x, y, and z directions, respectively.
When the first SAW is excited in the x-direction and the angular rate sensor is rotated about x-axis, the exciting force of the SAW acting on the metallic dots J11, J12, J13, J14, J15 due to the Coriolis force leads to excite the second SAW in the y directions. The second SAW generated by the Coriolis force acting on the metallic dots J11, J12, J13, J14, J15 is detected by the detecting IDTs J5, J6 since the detecting IDTs J5, J6 which are spaced with the distance of an integer number of the wavelength λ2 and have comb-shaped electrodes. The second SAW is strengthened being reflected back and forth by the reflectors J7 and J8 so that the second standing wave of the second SAW is generated efficiently. The reflectors J7 and J8 are located such that the detecting IDTs J5, J6 and the region J10 where the metallic dots J1 are disposed are sandwiched therebetween. The strength of the second standing wave is proportional to the Coriolis force. Therefore the strength of the second standing wave is proportional to the rotation rate of the angular rate sensor. Then the piezoelectric effect generates an electric field in the piezoelectric substrate proportional to the strength of the second standing wave which is detectable by the detecting IDTs J5, J6 as an electric voltage. Consequently, it is possible to obtain the rotation rate by measuring the voltage generated at the detecting IDTs J5, J6.
The elastic surface wave gyroscope of Kurosawa and Higuchi and the micro-electro-mechanical gyroscope of Varadan et al. are realized with technology for surface acoustic wave devices, especially, surface acoustic wave filters comprising a single crystal piezoelectric substrate for propagating a Rayleigh wave. Single crystal piezoelectric materials used as the piezoelectric substrate of the elastic surface wave gyroscope and the micro-electro-mechanical gyroscope includes but is not limited to, lithium niobate (LiNbO3), lithium tantalite (LiTaO3), lithium tetraborate (Li2B4O7).
However, even though the above mentioned single crystal piezoelectric materials such as lithium niobate (LiNbO3), lithium tantalite (LiTaO3), lithium tetraborate (Li2B4O7) are widely used for mechanical filters utilizing surface acoustic wave, it is not the best way to reduce the size of the device. Another option is to integrate an angular rate sensor and an external driving circuit thereof into an integrated device when the metallic dots J1 are disposed on one of the above single crystal piezoelectric materials for the angular rate sensor. However one disadvantage of the arrangement comes from the fact that high sensitivity of the angular rate sensor which needs a lot of individual metallic dots J1, and the relative positions of the metallic dots J1 need to be fine tuned so that every metallic dot is positioned at nodes of the secondary SAW caused by the Coriolis force. Furthermore, in order for the angular rate sensor to be highly sensitive, a necessary area of the region J10 where the metallic dots J1 are disposed must be larger in number. The mechanical structure of the elastic surface wave gyroscope of Kurosawa and Higuchi and the micro-electro-mechanical gyroscope of Varadan et al. is not suitable for reducing the size of the angular rate sensor and integrating an angular rate sensor and a driving circuit thereof into one integrated device.
Further, in a similar structure of the elastic surface wave gyroscope of Kurosawa and Higuchi and the micro-electro-mechanical gyroscope of Varadan et al., since it is necessary for the plurality of the metallic dots J1 to be located at the standing wave maxima in order to reduce transduction loss, each of the metallic dots J1 will never be too large or too heavy. However, small and light weight metallic dots J1 can not bring the angular rate sensor into having high sensitivity, because the Coriolis force is proportional to both mass and rotational rate.
In order to improve a downsizing achievement and the scale of integration of an angular rate sensor, an attempt has been made to fabricate on the semiconductor substrate the piezoelectric film in which the elastic acoustic waves are generated using the same principle as that of the elastic surface wave gyroscope of Kurosawa and Higuchi and the micro-electro-mechanical gyroscope of Varadan et al., as shown in FIG. 49. The angular rate sensor of this type comprises a semiconductor substrate J20, an insulator film J21, and a piezoelectric film J22. The insulator film J21 is deposited on an upper surface of the semiconductor substrate J20, and the piezoelectric film J22 is deposited on an upper surface of the insulator film J21. The metallic dots J23 for sensing the Coriolis force are deposited on an upper surface of the piezoelectric films.
However, the angular rate sensor of the type shown in FIG. 49 is not effective for following reasons. In the structure of the angular rate sensor of this type, an electric polarization in an accumulation direction in a z-direction or surface electric charges are caused when the surface acoustic wave is generated in the piezoelectric film. Wherein, the z-direction is defined as a direction perpendicular to a surface of the piezoelectric film. When the surface acoustic wave is generated, particles of the piezoelectric film vibrate in the z-direction due to the piezoelectric effect. Since the piezoelectric effect converts an inner stress of the piezoelectric film into an electric field, an amount of the electric polarization or an amount of electric charge are proportional to amplitudes of the elastic acoustic waves, then proportional to the stress applied to the piezoelectric film. If a piezoelectric constant is positive, positive electric charge is caused in a region at which a compression stress is generated, and negative electric charge is caused in other regions at which a tensile stress is applied. As a result of the electric polarization occurred in the piezoelectric film, a displacement of particles which are located in both regions at which the compression and tensile stresses are applied is suppressed. Then, achievement of high sensitivity of the angular rate sensor is difficult in the structure of the angular rate sensor of the type which is organized in such a way that the piezoelectric film J22 and the metallic dots J23 are accumulated on the semiconductor substrate J20.